TW202532979A - Focus metrology method and method for designing a focus target for focus metrology - Google Patents

Abstract

Disclosed is a method of metrology, comprising obtaining a first measurement parameter value relating to at least a first sub-target; obtaining a third measurement parameter value relating to at least a third sub-target; and determining at least a focus parameter from at least said first measurement parameter value, said determination being normalized using at least said third measurement parameter value. The at least third sub-target comprising a normalization sub-target comprising substantially no or minimal focus sensitivity. Also disclosed is a method for designing a focus target.

Description

Focusing metrology method and method for designing focusing targets for focusing metrology

The present invention relates to the application of metrology in integrated circuit manufacturing.

A lithography system is a machine designed to apply a desired pattern to a substrate. Lithography systems are used, for example, in the manufacture of integrated circuits (ICs). They can project a pattern (often referred to as a "design layout" or "design") from a patterned device (e.g., a photomask) onto a layer of radiation-sensitive material (resist) provided on a substrate (e.g., a wafer).

To project a pattern onto a substrate, lithography equipment uses electromagnetic radiation. The wavelength of this radiation determines the minimum size of features that can be formed on the substrate. Typical wavelengths currently used are 365 nm (i-line), 248 nm, 193 nm, and 13.5 nm. Compared to lithography equipment using radiation with a wavelength of, for example, 193 nm, lithography equipment using extreme ultraviolet (EUV) radiation with a wavelength in the 4 nm to 20 nm range (e.g., 6.7 nm or 13.5 nm) can be used to form smaller features on substrates.

Low- _k₁ lithography can be used to process features smaller than the classic resolution limit of the lithography equipment. In this process, the resolution formula can be expressed as CD = _k₁ × λ/NA, where λ is the wavelength of the radiation used, NA is the numerical aperture of the projection optics in the lithography equipment, CD is the "critical dimension" (usually the smallest printed feature size, but in this case, half-pitch), and _k₁ is an empirical resolution factor. Generally speaking, the smaller _k₁ , the more difficult it is to reproduce a pattern on the substrate that resembles the shape and size planned by the circuit designer to achieve specific electrical functionality and performance. To overcome these difficulties, complex fine-tuning steps can be applied to the lithography projection equipment and/or the design layout. These steps include, for example, but are not limited to, optimization of the NA, customizing the illumination scheme, using phase-shift patterning devices, various optimizations of the design layout such as optical proximity correction (OPC, sometimes also referred to as "optical and process correction") in the design layout, or other methods generally defined as "resolution enhancement technology" (RET). Alternatively, a strict control loop for controlling the stability of the lithography equipment can be used to improve the reproduction of the pattern at low k1.

Metrology tools are used in many aspects of the IC manufacturing process as, for example, alignment tools for proper positioning of substrates prior to exposure and as scatterometry-based tools for inspecting/measuring exposed and/or etched products in process control; for example, to measure overlays.

One important parameter of the lithography process that needs to be monitored is the focus. An ever-increasing number of electronic components needs to be integrated into ICs. In order to achieve this integration, it is necessary to reduce the size of the components and therefore increase the resolution of the projection system so that smaller and smaller details or line widths can be projected onto the target portion of the substrate. As the critical dimension (CD) in lithography shrinks, the consistency of the focus across the substrate and between substrates becomes increasingly important. The CD is the dimension of a feature (e.g. the gate width of a transistor) whose variation will cause undesirable variations in the physical properties of one or more features. Traditionally, the optimal settings are determined by "wafer advance", that is, substrates that are exposed, developed and measured before the production run. In advance shipment of wafers, test structures are exposed in a so-called focus energy matrix (FEM), and the optimal focus and energy settings are determined based on inspection of these test structures.

Another method of determining focus and/or dose has been through diffraction-based focus techniques. Diffraction-based focus can use target-forming features on a zoom mask that print a target with a degree of asymmetry that depends on the focus and/or dose settings during printing. This degree of asymmetry can then be measured and the focus and/or dose inferred based on the asymmetry measurement. These focus measurement methods and the associated test structure designs have several disadvantages. Many test structures require sub-resolution features or grating structures with large pitches. These structures can violate the design rules of the user of the lithography equipment. Focus metrology techniques may involve measuring asymmetries in relatively high-order (e.g., first-order) radiation scattered by a particular focus-dependent target structure and determining focus based on the asymmetry.

There is a need to improve current focus metrology techniques, especially in the context of EUV lithography.

In a first aspect of the present invention, a metrology method is provided, comprising: obtaining a first measurement parameter value associated with at least a first sub-target; obtaining a third measurement parameter value associated with at least a third sub-target; and determining at least one focus parameter based on at least the first measurement parameter value, wherein the determination is normalized using at least the third measurement parameter value, wherein the at least third sub-target includes a normalized sub-target, wherein the normalized sub-target includes substantially no focus sensitivity or includes minimal focus sensitivity.

In a second aspect of the present invention, a method for designing a focus target is provided, comprising: obtaining a pool of candidate sub-targets; obtaining measurement parameter data, the measurement parameter data comprising respective measurement parameter values for each candidate sub-target in the candidate sub-target pool within a focus range of interest and a dose range of interest; fitting the measurement parameter values in focus and dose for each candidate sub-target based on a plurality of fitting coefficients; determining a plurality of difference coefficients for different pairs of the candidate sub-targets based on the plurality of fitting coefficients; and evaluating the plurality of difference coefficients relative to one or more criteria to determine one or more feasible targets, each feasible target comprising a feasible combination of candidate sub-targets.

In this document, the terms "radiation" and "beam" are used to cover all types of electromagnetic radiation, including ultraviolet radiation (e.g., having a wavelength of 365, 248, 193, 157, or 126 nm) and EUV (extreme ultraviolet radiation, e.g., having a wavelength in the range of about 5 to 100 nm).

As used herein, the terms "reduction reticle," "mask," or "patterning device" should be broadly interpreted to refer to a general-purpose patterning device that can be used to impart a patterned cross-section to an incident radiation beam, corresponding to the pattern to be produced in a target portion of a substrate. In this context, the term "light valve" may also be used. In addition to classic masks (transmissive or reflective; binary, phase-shift, hybrid, etc.), other examples of such patterning devices include programmable mirror arrays and programmable LCD arrays.

FIG1 shows a lithography system including a radiation source SO and a lithography apparatus LA. The radiation source SO is configured to generate an EUV radiation beam B and to supply the EUV radiation beam B to the lithography apparatus LA. The lithography apparatus LA includes an illumination system IL, a support structure MT configured to support a patterning device MA (e.g., a reticle), a projection system PS, and a substrate table WT configured to support a substrate W.

The illumination system IL is configured to condition the EUV radiation beam B before the EUV radiation beam B is incident on the patterning device MA. Furthermore, the illumination system IL may include a faceted field mirror device 10 and a faceted pupil mirror device 11. Together, the faceted field mirror device 10 and the faceted pupil mirror device 11 provide the EUV radiation beam B with a desired cross-sectional shape and a desired intensity distribution. The illumination system IL may also include other mirrors or devices in addition to or in place of the faceted field mirror device 10 and the faceted pupil mirror device 11.

After being conditioned in this manner, EUV radiation beam B interacts with patterning device MA. This interaction results in a patterned EUV radiation beam B'. A projection system PS is configured to project patterned EUV radiation beam B' onto substrate W. For this purpose, projection system PS may include a plurality of mirrors 13, 14 configured to project patterned EUV radiation beam B' onto substrate W held by substrate table WT. Projection system PS may apply a reduction factor to patterned EUV radiation beam B', thereby forming an image having features that are smaller than corresponding features on patterning device MA. For example, a reduction factor of 4 or 8 may be applied. Although the projection system PS is depicted in FIG. 1 as having only two mirrors 13 , 14 , the projection system PS may include a different number of mirrors (eg, six or eight mirrors).

The substrate W may include a previously formed pattern. In this case, the lithography apparatus LA aligns the image formed by the patterned EUV radiation beam B′ with the pattern previously formed on the substrate W.

A relative vacuum, i.e. a small amount of gas (e.g. hydrogen) at a pressure substantially below atmospheric pressure, may be provided in the radiation source SO, in the illumination system IL and/or in the projection system PS.

The radiation source SO shown in FIG1 is of a type that can be referred to as a laser-produced plasma (LPP) source, for example. A laser system 1, which can include, for example, a _CO2 laser, is configured to deposit energy via a laser beam 2 into a fuel, such as tin (Sn), provided by, for example, a fuel emitter 3. Although tin is mentioned in the following description, any suitable fuel can be used. The fuel can be, for example, in liquid form and can be, for example, a metal or an alloy. The fuel emitter 3 can include a nozzle configured to guide tin, for example, in the form of droplets, along a trajectory toward a plasma formation region 4. The laser beam 2 is incident on the tin at the plasma formation region 4. Depositing the laser energy into the tin generates a tin plasma 7 at the plasma formation region 4. During deexcitation and recombination of electrons and plasma ions, radiation including EUV radiation is emitted from the plasma 7.

EUV radiation from the plasma is collected and focused by a collector 5. The collector 5 comprises, for example, a near-normal-incidence radiation collector 5 (sometimes more generally referred to as a normal-incidence radiation collector). The collector 5 may have a multi-layer mirror structure configured to reflect EUV radiation (e.g., EUV radiation having a desired wavelength, such as 13.5 nm). The collector 5 may have an ellipsoidal configuration with two focal points. As discussed below, a first of these focal points may be located at the plasma formation region 4, and a second of these focal points may be located at an intermediate focus 6.

Laser system 1 can be spatially separated from radiation source SO. In this case, laser beam 2 can be transferred from laser system 1 to radiation source SO by means of a beam delivery system (not shown) comprising, for example, suitable steering mirrors and/or beam expanders and/or other optical components. Laser system 1, radiation source SO, and beam delivery system can be collectively referred to as a radiation system.

The radiation reflected by the collector 5 forms an EUV radiation beam B. The EUV radiation beam B is focused at a central focus 6 to form an image at the central focus 6 of the plasma present in the plasma formation region 4. The image at the central focus 6 serves as a virtual radiation source for the illumination system IL. The radiation source SO is configured so that the central focus 6 is located at or near an opening 8 in an enclosure 9 of the radiation source SO.

Although FIG1 depicts the radiation source SO as a laser produced plasma (LPP) source, any suitable source such as a discharge produced plasma (DPP) source or a free electron laser (FEL) may be used to generate EUV radiation.

As shown in FIG2 , the lithographic apparatus LA may form part of a lithographic cell LC (sometimes also referred to as a lithocell or a (lithographic) cluster), which often also includes equipment for performing pre-exposure and post-exposure processes on a substrate W. As is known, this equipment includes a spin coater SC for depositing a resist layer, a developer DE for developing the exposed resist, a cooling plate CH, and a baking plate BK. The cooling plate CH and baking plate BK are used, for example, to regulate the temperature of the substrate W, such as for regulating the solvent in the resist layer. A substrate handler or robot RO picks up substrates W from input/output ports I/O1 and I/O2, moves the substrates W between various process equipment, and delivers the substrates W to the loading area LB of the lithography apparatus LA. The equipment in the lithography unit, often collectively referred to as the coating and development system, is typically controlled by a coating and development system control unit (TCU). The coating and development system control unit itself may be controlled by a supervisory control system (SCS), which may also control the lithography apparatus LA, for example, via the lithography control unit (LACU).

To correctly and consistently expose substrates W exposed by lithography apparatus LA, the substrates need to be inspected to measure properties of the patterned structures, such as overlay errors between subsequent layers, line thickness, critical dimensions (CDs), and so on. For this purpose, an inspection tool (not shown) may be included in lithography cell LC. If errors are detected, adjustments can be made to the exposure of subsequent substrates or to other processing steps to be performed on substrate W, particularly before other substrates W from the same batch or lot are exposed or processed.

The inspection equipment, which may also be referred to as metrology equipment, is used to determine properties of the substrate W, and more specifically, to determine how the properties vary between different substrates W or how properties associated with different layers of the same substrate W vary from layer to layer. The inspection equipment may alternatively be configured to identify defects on the substrate W and may, for example, be part of the lithography cell LC, may be integrated into the lithography apparatus LA, or may even be a standalone device. The inspection equipment can measure properties on a latent image (the image in the resist layer after exposure), a penumbra image (the image in the resist layer after the post-exposure bake (PEB) step), a developed resist image (where either the exposed or unexposed portions of the resist have been removed), or even an etched image (after a pattern transfer step such as etching).

Typically, the patterning process in a lithography apparatus LA is one of the most critical steps in the process, requiring high accuracy in the dimensioning and placement of structures on the substrate W. To ensure this high accuracy, three systems can be combined in a so-called "holistic" control environment, as schematically depicted in FIG3 . One of these systems is the lithography apparatus LA, which is (in practice) connected to a metrology tool MT (a second system) and to a computer system CL (a third system). The key to this "holistic" environment is to optimize the cooperation between these three systems to enhance the overall process window and provide a tight control loop, thereby ensuring that the patterning performed by the lithography apparatus LA remains within the process window. The process window defines a range of process parameters (e.g., dose, focus, overlay) within which a specific fabrication process produces a defined result (e.g., a functional semiconductor device)—typically allowing process parameter variations in a lithography process or patterning process.

Computer system CL can use (a portion of) the design layout to be patterned to predict which resolution enhancement technology to use and perform computational lithography simulations and calculations to determine which mask layout and lithography tool settings achieve the maximum overall process window for the patterning process (depicted in FIG3 by the double arrows in the first scale SC1). Typically, the resolution enhancement technology is configured to match the patterning capabilities of lithography tool LA. Computer system CL can also be used to detect where within the process window lithography tool LA is currently operating (e.g., using input from metrology tool MT) to predict whether defects are present, for example, due to suboptimal processing (depicted in FIG3 by the arrow pointing to "0" in the second scale SC2).

The metrology tool MT may provide input to the computer system CL to enable accurate simulation and prediction, and may provide feedback to the lithography apparatus LA to, for example, identify possible drifts in the calibration state of the lithography apparatus LA (depicted in FIG. 3 by arrows in the third scale SC3).

In lithography processes, measurements of the produced structures frequently need to be performed, for example, for process control and verification. The tool used to perform such measurements is usually called a metrology tool MT. Different types of metrology tools MT for performing such measurements are known, including scanning electron microscopes or various forms of scatterometer metrology tools MT. Scatterometers are versatile instruments that allow the measurement of parameters of the lithography process either by having a sensor in the pupil or in a plane conjugated to the pupil of the objective lens of the scatterometer (the measurement is usually called pupil-based metrology) or by having a sensor in the image plane or in a plane conjugated to the image plane, in which case the measurement is usually called image- or field-based metrology. Such scatterometers and related measurement techniques are further described in patent applications US20100328655, US2011102753A1, US20120044470A, US20110249244, US20110026032, or EP1,628,164A, which are incorporated herein by reference in their entirety. These scatterometers can measure gratings using light from soft x-rays and light visible to the near IR wavelength range.

When monitoring a lithography process, it is necessary to monitor lithography process parameters, such as the focus parameter, which describes the focus of the lithography beam on the substrate. One known method for determining the focus parameter from a printed structure is by micro-diffraction-based focusing (μDBF). Micro-diffraction-based focusing can use target-forming features on a magnification reticle that print a target with a degree of asymmetry that depends on the focus setting during printing. This degree of asymmetry can then be measured using a scatterometry-based detection method, for example by measuring the intensity asymmetry between the intensities of the +1st-order radiation and the -1st-order radiation diffracted from the target, to obtain a measure of the focus setting. Focus-dependent asymmetry can be achieved by providing "sub-resolution features" (also called sub-resolution auxiliary features) that are below the resolution limit of the lithography tool being used to expose the target. Because these sub-resolution features are below this resolution limit, they are not actually formed on the substrate; instead, they affect the formation of other (larger) neighboring features that are resolved. Specifically, by placing these sub-resolution features along only one wall of a larger feature, the wall formation (e.g., sidewall angle) of this wall will depend on the focus. In this way, the symmetry (or asymmetry) of the larger feature will be focus-dependent.

This μDBF method is generally effective for DUV focus parameter monitoring. However, in the case of EUV lithography equipment, the higher resolution of the lithography equipment allows smaller features on the target (such as auxiliary features) to be resolved, thereby weakening the signal from such targets. Furthermore, EUV lithography uses thinner resists, which also means that the signal is generally weaker for EUV μDBF than for DUV μDBF. To address the signal strength issue, an alternative EUV focus metrology technique, hereinafter referred to as feature-biased focus metrology, may use a target comprising a pair of focus sub-targets, wherein each sub-target has a well-known Bossung curve relationship with focus, i.e., comprising a respective Bossung curve peaking at best focus, and wherein each of the sub-targets may form Bossung curves having respective different best focus values and partially overlapping. Optionally, each of the Bossung curve relationships may be substantially similar except for the best focus offset.

A metrology technique may employ a target comprising at least a first sub-target and a second sub-target, wherein the first sub-target and the second sub-target are configured to have an optimal focus offset df between the two sub-targets. For each of the first sub-target and the second sub-target, a focusing response to a measured value of a sub-target parameter may take the form of a Bersong curve. The focus varies depending on a first measured value of the sub-target parameter obtained from a measurement of the first sub-target and a second measured value of the sub-target parameter obtained from a measurement of the second sub-target. Therefore, it is proposed to obtain measured values of the parameters from the first sub-target and the second sub-target and derive a focus value from these measured values. The measured parameter may be an intensity parameter, such as the intensity of at least one diffraction order (e.g., the zeroth order and/or one or more first-order diffraction orders). A monotonic relationship may be obtained by comparing (eg, the difference) the measurement from the first sub-goal to the measurement from the second sub-goal (and optionally dividing by the average measurement of the two sub-goals).

In one embodiment, an optimal focus shift can be achieved through induced astigmatism, for example using an astigmatism-based focusing (ABF) technique. Astigmatism can be induced in the imaging lens and/or the refraction mask. ABF is described in US2016363871A1, which is incorporated herein by reference. Induced astigmatism can be nonideal, making it unusable for production monitoring. Therefore, other methods can be used to achieve a suitable optimal focus shift, for example, based on the difference in target parameters between the first and second sub-targets.

To improve EUV layer focus performance, one approach can involve printing a large number (e.g., hundreds or thousands) of focus targets on the EUV wafer and measuring all of them to find the best-performing target. This brute-force approach doesn't always guarantee a good focus target design and is time-consuming and expensive. Another approach can optimize the current DUV target design for each EUV layer. However, this is extremely slow and, likewise, isn't guaranteed to provide a satisfactory target.

Many of the metrology techniques described above involve measuring first order signals and looking at the differences between them. Therefore, they do not guarantee intensity matching between different metrology tools. Metrology tools may be affected by variations in dose (exposure apparatus source intensity) between tools and/or within a single tool over time (e.g., during exposure of a substrate). Therefore, the measured signal may be normalized to achieve tool matching. Now, for focus metrology, the same targets used to generate focus sensitivity are also used to normalize the measured signal. For (μ)ABF and feature bias focus metrology, the parameter value or intensity metrology asymmetry (intensity asymmetry) between the first sub-target T1 and the second sub-target T2 is measured Normalized by the sum strength measure (sum strength) of the two objectives: Each of the measured strength metrics may optionally comprise the average of the strengths of each of the two complementary diffraction orders (e.g., +1 order and -1 order) from the respective targets. For (μ)DBF, this means the strength metric asymmetry or strength asymmetry between the order 1s of the μDBF subtargets. Normalized by the sum of two order 1 strengths: .

However, because the resist thickness is small for EUV lithography, the signal intensity/diffraction efficiency (DE) of the measured target is low. This low DE means that the current sensor calibration is not effective enough in calibrating the intensity error. If the signal is normalized by the summed intensity, any consistent error in the measured intensity is included in the measured asymmetry, for example (in the context of feature bias focus metrology):

To reduce sensitivity to errors in measured intensity, one option is to increase the focus sensitivity of the target. Thus, the relative (focus) impact of intensity errors will become smaller. In focus metrology, finding a good target is often a balance between sensitivity to scanner focus and dose/energy. Many current focus-sensitive targets also exhibit a large sensitivity to dose, and many current target selection strategies search for target combinations that largely cancel out the dose sensitivity. However, the normalized asymmetry formula restricts this behavior because the normalization may involve dividing by a sum so that the dose sensitivity is still included in the measured asymmetry, for example: .

It may also be desirable to untangle focus and dose sensitivity and/or to measure (scanner) focus and (scanner) dose separately. This allows using the measured focus and dose values to improve control of focus and dose during exposure.

Ghost or stray light calibration, for example, calibration for "ghost" or stray light within the sensor optics, should be performed on (e.g., production) wafers. However, there may not be dedicated space on the production wafer for this ghost calibration, and metrology tool fiducials are used instead to approximate the error. This can be inaccurate and even increase the error offset.

The concepts disclosed herein relate to methods for focusing (and optionally dosing) metrics and/or methods for target selection for focusing (and optionally dosing) metrics. Specifically, the methods disclosed herein can address one or more of the problems described above.

In one embodiment, the method may include providing one or more normalization sub-targets and using measurements from these normalization sub-targets to normalize focus (and optionally dose) measurements. One or more normalization sub-targets may be dedicated normalization sub-targets, such that measurement data (e.g., measured signals) from the normalization sub-target(s) are used solely for normalization. Each of these one or more normalization sub-targets may include a design that has no sensitivity or minimal sensitivity (e.g., below a threshold focus sensitivity) to scanner focus. One or more normalization sub-targets may also include no sensitivity or minimal sensitivity (e.g., below a threshold dose sensitivity) to scanner dose. This may be the case where only a single normalization sub-target is provided. Alternatively or additionally, in the case of providing a pair of normalized sub-targets, they may have substantially the same dose sensitivity, such that a signal difference may be obtained on the two targets, wherein the dose sensitivity cancels out. In either case, the focus threshold may be set with reference to the focus sensitivity of one or more focus sensitivity targets or the focus sensitivity of their differences. For example, the maximum threshold may be obtained by dividing the denominator (e.g., ) relative to the molecule In another embodiment, the focus and/or dose thresholds may include normalized second difference coefficient thresholds. and the normalized third difference coefficient threshold This will be described in more detail later.

One or more normalization sub-targets may be placed adjacent to one or more focus-sensitive sub-targets within a single target, allowing the entire target to be measured in a single capture.

Figure 4(a) illustrates an exemplary target design for feature bias or ABF metrology. The target includes a first sub-target, T1, and a second sub-target, T2, e.g., two focus-sensitive sub-targets (e.g., including an optimal focus offset between them, as already described). The target further includes at least one third sub-target, or normalization sub-target, and in this particular example, two third sub-targets, or normalization sub-targets, T3 and T4. Figure 4(b) illustrates another target design in which the regions of the first and second sub-targets, or focus-sensitive sub-targets, T1 and T2, are partitioned to accommodate the third/normalization sub-target, T3 and T4, between them. Other layouts are possible. For example, the disclosed concepts can be used with μDBF targets in μDBF metrology (in this case, only one focus-sensitive sub-target is strictly required, but two mirror sub-targets may be preferred).

These normalization sub-objectives enable the normalized focus-sensitive asymmetry signal to be determined as (e.g., in the feature bias example): Where T1 , T2 are the respective intensities of the two focus-sensitive sub-targets, and T3 , T4 are the respective intensities of the two normalized sub-targets. Assuming that all sub-targets are measured in a single acquisition, the uniformity error will be automatically calibrated: .

Similarly, in the case where the method includes μDBF metrology techniques, the normalized focus-sensitive asymmetry signal can be calculated as, for example: Given two focus-sensitive sub-targets and two normalized sub-targets, their asymmetry measures can be combined.

In one embodiment, two normalized sub-targets may be selected such that each has a dose sensitivity similar to that of the two focus-sensitive sub-targets in order to more effectively cancel dose crosstalk: .

Because this method achieves calibration of the dose perturbations of the focus sub-targets, it also achieves both focus metrology and dose metrology. By combining these targets with the learned calculation of the focus sensitivity asymmetry (which is sensitive to dose), e.g. , both focus information and dosage information can be extracted.

A more robust focusing and dosing metrology method and target selection method will now be described, which also utilizes normalized sub-targets. This method can use targets such as those shown in Figure 5, which include a first sub-target B, a second sub-target C, a third sub-target D, and a fourth sub-target A. Again, the targets can be configured to be captured in a single measurement acquisition.

The described concept can be based on the concept of feature bias and thus can include largely or substantially symmetric sub-targets (e.g., all sub-targets can be substantially symmetric). The sub-targets can include two substantially focus-sensitive sub-targets: a first sub-target B and a second sub-target C, and two substantially focus-insensitive sub-targets: a third sub-target D and a fourth sub-target A. The targets can be configured to generate first and second focus-sensitive signals. In this method, it is proposed that the first focus-sensitive (and dose-sensitive) signal Signal ₁ and the second focus-sensitive (and dose-sensitive) signal Signal ₂ can be defined as: , in 、 、 、 The fourth measurement parameter value, the first measurement parameter value, the second measurement parameter value, and the third measurement parameter value or intensity metric from sub-targets A , B , C , and D , respectively. The intensity metric may include at least one order of intensity (e.g., the zero order of radiation scattered by each sub-target or a diffracted non-zero order). In one embodiment, each of the measured intensity metrics may include the average of the intensities of two complementary diffraction orders (e.g., the +1 diffraction order and the -1 diffraction order) of radiation diffracted from the respective sub-target, for example: .

In this way, a first signal may be the difference of measurement parameters such as the intensity metric from the fourth sub-target A and the first sub-target B (e.g., A-B), and a second signal may be the difference of measurement parameters such as the intensity metric from the fourth sub-target A and the second sub-target C (e.g., A-C), each signal normalized by the difference of the measurement/intensity metric from the fourth sub-target A and the measurement/intensity metric from the third sub-target D. In all cases, the presence of the measurement of the fourth sub-target A accounts for stray light offsets, and the third sub-target D may be the normalizing sub-target.

In this method, calibration can be performed in a calibration or recipe optimization phase, which calibrates the first signal Signal ₁ and the second signal Signal ₂ with varying doses and varying focuses (e.g., within a dose range of interest and a focus range of interest) to obtain optimized signal data. (That is, the first optimized signal data and the second optimized signal data ). For example, optimizing signal data May include measured and Through 2D fitting of dose and focus, in the production or monitoring phase, the measurement or monitoring signal data can then be minimized. and optimized signal data Comparison of focus and dose to infer dose and focus: This minimization may include finding at least one local error minimum with a desired trend, for example, around expected focus and/or dose values and/or within an expected range of focus and dose. This may be done for one or more measurement conditions (e.g., wavelength and polarization combinations). For example, and Can be associated with more than one measurement condition.

A target design method for selecting each of subtargets A, B, C, and D from a library of candidate subtarget designs will now be described. The candidate subtargets may comprise a design of the general form depicted in FIG6 . In FIG6( a ), the target may comprise segmented features at an optical spacing _Pop detectable by metrology tools. The segmentation of the features may form a segment spacing (or a quasi-product spacing, e.g., a spacing having a size similar to that of a product feature) P _p , where each segmented subfeature has a quasi-product critical dimension (CD) CD _p . FIG6( b ) shows a similar configuration, but where each segmented subfeature is further subsegmented in the y-direction to simulate a contact hole configuration. Figure 6(c) shows a similar configuration again, but with a periodic y-offset between adjacent segmented sub-features, resulting in a tilted or inclined segment spacing _Pp . The segment spacing _Pp can be, for example, 30, 45, or 60 degrees.

The candidate sub-target library may include many variations of these (and other) designs, wherein one or more of the following, in particular, the optical pitch, the segment pitch in x and/or y, the segment y offset, and the segment CD may vary throughout the candidate sub-target library. For example, the number of candidate sub-targets in the library may be greater than 40, greater than 50, greater than 60, greater than 70, greater than 80, or greater than 90.

FIG7 is a flow chart describing the proposed target design method. A library of candidate sub-targets 700 is obtained (e.g., virtually, i.e., in simulation), and the library of candidate sub-targets is measured (e.g., again virtually or in simulation) to obtain a respective measurement parameter or intensity metric value for each candidate sub-target for different values of a focus metric (e.g., scanner focus) and a dose metric (e.g., scanner dose). The obtained measurement parameter data or intensity metric data can optionally be further correlated with one or more measurement conditions (e.g., different wavelengths and/or polarizations).

In step 705, the strength metric is fitted to a 2D curve to obtain the strength of each candidate sub-target. The fitting can be done based on six fitting coefficients (a constant coefficient , a dose factor , a focusing factor , focused dose product coefficient , a focusing square coefficient and a focused squared dose product coefficient ), for example: Where m is the index of different candidate sub-goals.

At step 710, the difference coefficients of the difference signal are determined. For all unique dual candidate sub-goal combinations (e.g., about 5000 for about 100 candidate sub-goals), a first difference coefficient [Unit: diffraction efficiency DE (in simulation) or gray level GL (in reality)], a second difference coefficient [Unit: DE/nm or GL/nm] and a third difference coefficient [Unit: DE/nm ² or GL/nm ² ] can be determined as: in represents the nominal scanner exposure dose, and represents the combination of a first candidate sub-target m and a second candidate sub-target k .

At step 715, the target combination of the fourth sub-target (sub-target A) and the combination of the first sub-target (sub-target B) and the second sub-target (sub-target C) is determined so that it generates Signal ₁ and Signal ₂ with desired characteristics. A desired characteristic can be a combination of 、 The difference between each combination of is sufficient to make the sub-target combination have different tilts through focusing (where m , k and k ' are indexed for the fourth sub-target, the first sub-target and the second sub-target respectively). Tilt can be referred to as the second difference coefficient (Linear coefficient) relative to the third difference coefficient (quadratic coefficient). For example, if , then the tilt through focusing is stronger than the curvature through focusing. Another desirable characteristic may be the combination of 、 The combinations have roughly the same strength.

Thus, this step 715 may include finding, for each fourth subgoal design, a unique combination of k and k' that satisfies the following equation: About The check ensures that both target combinations have (approximately) the same strength. Check to ensure that the target combination has different tilts through focus (e.g., sufficiently large optimal focus offset). is the first difference coefficient threshold, and is the second difference coefficient threshold.

At step 720, candidate sub-goals to be used for normalization are determined. The result of the previous step 715 is a list of suitable combinations of candidate sub-goals for the fourth sub-goal, the first sub-goal, and the second sub-goal (e.g., suitable first and second sub-goals for each suitable fourth sub-goal). This step aims to find a third sub-goal or normalization sub-goal (sub-goal D) for each fourth sub-goal A that meets the following criteria: [To ensure not to divide by too small a number] and [to ensure regularity through adequate consistency of dosage/focus] [To ensure that normalization is sufficiently constant through dose/focus] where m is the index of the fourth sub-goal (Goal A) and k is the index of the third sub-goal (Goal D). is the normalized first difference coefficient threshold, is the normalized second difference coefficient threshold, and is the normalization third difference coefficient threshold. This generates a list of candidate subgoals to be used for each subgoal A for regularization.

At step 725, the results of steps 715 and 720 are combined and key performance indicators (KPIs) can be evaluated to identify one or more feasible combinations, for example, one or more feasible four-subgoal combinations that can be used as goals that can be referred to as feasible goals. Step 715 generates a list of feasible candidate subgoal combinations of the first and second subgoals for each candidate fourth subgoal, and step 720 generates a list of feasible candidate third (normalized) subgoals for each fourth subgoal. Feasible in this context describes subgoals and combinations thereof that meet the criteria outlined in steps 715 and 720.

Step 725 can be further divided into the flow chart of Figure 8. At step 800, all candidates for feasible goals are determined and constructed (e.g., depending on whether this step is simulated or modeled or exposed based on actual measurements), that is, feasible candidate goals; that is, all combinations of sub-goals (of the four sub-goals A, B, C, D) that meet the criteria outlined in steps 715 and 720. At step 805, an optimization signal can be generated within the focus range and dose range of interest. , where n = 1 and 2 (i.e., the first signal and the second signal as already described). At step 810, the monitoring signal may be calculated on the 2D focus/dose grid. At step 815, the error signal derived from the difference between the optimization signal and the monitoring signal is minimized to find the focus value and dose value focus _get , dose _get for each feasible candidate target. This can be done using the method described above. At step 820, focus sensitivity FS and dose sensitivity DS can be calculated based on the acquired values and the corresponding set values (i.e., as set in simulation or on the scanner). Focus sensitivity FS and dose sensitivity DS can be determined based on the relationship between the signal data and focus and dose; this describes how the determined signal changes with focus and/or dose. At step 825, each candidate target can be evaluated based on the following KPIs, for example: maximum dose/focus setting acquisition error on the 2D grid and minimum DS/FS on the 2D grid.

The goals you choose at this step can meet the following criteria: in Set error thresholds for dose acquisition. Set the error threshold for focus acquisition. is the focus sensitivity threshold, and This is the dose sensitivity limit.

The actual target selected may be the best compromise and/or optimal compromise of these three criteria, the best performance target for all three criteria (if any), or any target that satisfies these criteria (possibly taking into account one or more secondary criteria related to, for example, design rules or optimal configurations). Optionally, dose sensitivity and/or dose error are evaluated depending on the importance of the dose metrics (i.e., the extrapolated fit described above may be performed only for focus using nominal dose values). It is possible that these criteria may have different importance when selecting a compromise, for example, focus sensitivity may be considered the most important and prioritized (e.g., the target selected may be the most focus-sensitive of the targets that meet the relevant criteria).

Any of the simulations described above can be performed using a computational lithography module capable of simulating a lithography process for transferring a patterned device pattern onto a resist layer of a substrate and a computational metrology module capable of simulating metrology operations on the patterned device pattern.

In some embodiments, the optimization process of the system can be expressed as a cost function. The optimization process can include finding a set of parameters of the system (design variables, process variables, detection operation variables, etc.) that minimizes the cost function. The term "design variables" as used herein includes a set of parameters of the lithography projection device or the lithography process, for example, user-adjustable parameters of the lithography projection device, or image characteristics that the user can adjust by adjusting those parameters. It should be understood that any characteristics of the lithography projection process (including characteristics of the source, patterning device, projection optical components, or anti-etching agent characteristics) can be among the design variables in the optimization. The cost function can have any suitable form depending on the goal of the optimization. For example, the cost function can be the weighted root mean square (RMS) of the deviation of certain characteristics (evaluation points) of the system relative to the expected values of these characteristics (e.g., ideal values). The cost function can often be a nonlinear function of the design variables. Standard optimization techniques can then be used to minimize the cost function.

Another embodiment is a method for designing a focus target. The method includes the following steps: 1) obtaining a library of candidate sub-targets; 2) obtaining measurement parameter data for each candidate sub-target in the library within a desired focus range and a desired dose range; 3) fitting the measurement parameter data in terms of focus and dose for each candidate sub-target; and 4) determining one or more targets, each target comprising a combination of candidate sub-targets.

Another embodiment is a metrology target for determining a parameter of a lithography process, optionally the parameter being focus (or optionally focus and dose) of the lithography process. The target includes at least a first sub-target, a second sub-target, a third sub-target, and a fourth sub-target. Each of the sub-targets has a different design than the other targets. Optionally, the sub-targets are largely or substantially symmetric (e.g., all sub-targets may be substantially symmetric). The sub-targets may include two sub-targets that are substantially sensitive to the parameter, such as a first sub-target B and a second sub-target C, and two sub-targets that are substantially insensitive to the parameter, such as a third sub-target D and a fourth sub-target A. Parameters of the lithography process are determined based on the measurement parameter data of the first sub-target, the second sub-target, the third sub-target, and the fourth sub-target.

Another embodiment provides a method for designing a target for measuring a first parameter, wherein the target includes multiple sub-targets. The method includes obtaining a library of candidate sub-targets as described in the above embodiments. The method may include obtaining measurement parameter data, wherein the measurement parameter data includes respective measurement parameter values for each candidate sub-target in the library within a range of interest for the first parameter and a range of interest for a second parameter. The measurement parameter data may include a matrix, and the matrix may be a 2D matrix. The matrix may vary depending on the first parameter and the second parameter. The method may include determining one or more feasible goals, wherein each feasible goal includes a feasible combination of candidate sub-goals such that the matrix has a single global minimum or maximum value within a range of interest for the first parameter and a range of interest for the second parameter. The goal may be used to measure both the first parameter and the second parameter. The measured parameter data may include a first measured parameter value associated with at least a first sub-goal and a second measured parameter value associated with at least a second sub-goal. For example, the first sub-goal is sub-goal T1 or sub-goal B , and the second sub-goal is sub-goal T2 or sub-goal C. The first parameter may be determined based on a difference and/or comparison between the first measured parameter value and the second measured parameter value. The target may further include a third sub-target and the measurement parameter data, the measurement parameter data including a third measurement parameter value associated with at least the third sub-target. For example, the third sub-target is sub-target T3 or sub-target D. The target may further include a fourth sub-target and the measurement parameter data including a fourth measurement parameter value associated with at least the fourth sub-target. For example, the fourth sub-target is sub-target T4 or sub-target A. The sub-targets may include two sub-targets that are substantially sensitive to the first parameter and two sub-targets that are substantially insensitive to the first parameter. The first parameter may be a parameter of a lithography process, such as a focus parameter. The method may further include using a computational lithography module capable of simulating a lithographic process for transferring a patterned device pattern onto a resist layer of a substrate, and a computational metrology module capable of simulating metrological operations on the patterned device pattern. The patterned device pattern may be used to pattern a target. The second parameter may be a dose parameter. Each of the sub-targets may be substantially symmetric. Each of the sub-targets may have a design different from that of the other targets. All sub-targets may be measured in a single acquisition. Each measured parameter value may include an intensity metric. The intensity metric may include an average of respective intensities of complementary diffraction orders from each of the sub-targets. Parameters other than focus and/or dose, such as resist thickness and/or target top surface roughness, should not affect the single global minimum or maximum focus and dose values and/or should not affect the single global minimum or maximum within the range of interest for focus and dose.

Another embodiment is a method for inferring a value of a first parameter (e.g., a focus parameter) and a value of a second parameter (e.g., a dose parameter of a lithography process). The method includes the steps of determining a first metric and a second metric based on measurement data associated with a target formed on a substrate using the lithography process. Each of the first metric and the second metric depends on both a first processing parameter and a second processing parameter. The first metric has a different dependency on the first processing parameter than the second processing parameter, and the second metric has a different dependency on the second processing parameter than the first processing parameter. The method further includes inferring the value of the first parameter and the value of the second parameter based on the first metric and the second metric. The target does not include any asymmetric features, such as an asymmetric resist profile resulting from, for example, a high-resolution substructure. All features of the target, such as the resist profile, are symmetric. The second metric may comprise a sum metric of the sum of the intensities of complementary diffraction orders based on diffraction from the measurement radiation following the measurement target. In one example, the target comprises two or more sub-targets that are selected to be exposed on a substrate for use in determining focus parameters associated with focus settings of a lithography exposure process in which the two or more sub-targets are exposed. The lithography exposure process may comprise using extreme ultraviolet radiation for exposure. The method may further comprise selecting a first sub-target comprising a first only symmetric feature, and a second sub-target comprising a second only symmetric feature. The first only symmetric feature and the second only symmetric feature may have different respective best focus values. The first metric and/or the second metric may be an intensity metric asymmetry (intensity asymmetry) between the first sub-target T1 and the second sub-target T2 The first metric and/or the second metric may be a sum strength metric of the two objectives (sum strength): Each of the measured intensity metrics may optionally comprise an average of the intensities of each of two complementary diffraction orders (e.g., +1 order and -1 order) from each respective target.

Another embodiment is a non-transitory computer program product comprising machine-readable instructions that, when executed by a computer system, are configured to cause the computer system to at least cause the method of any one of the above embodiments to be performed.

Further embodiments are disclosed in the following numbered clauses: 1. A metrology method, comprising: obtaining a first measurement parameter value associated with at least one first sub-target; obtaining a third measurement parameter value associated with at least one third sub-target; and determining at least one focus parameter based on at least the first measurement parameter value, wherein the determination is normalized using at least the third measurement parameter value, the at least third sub-target comprising a normalized sub-target, the normalized sub-target comprising substantially no focus sensitivity or comprising minimum focus sensitivity. 2. The method of clause 1, further comprising obtaining a second measurement parameter value associated with at least one second sub-target; and determining at least one focus parameter based on the first measurement parameter value and the second measurement parameter value. 3. The method of clause 2, wherein the focus parameter is determined based on a difference and/or comparison between the first measurement parameter value and the second measurement parameter value. 4. The method of clause 2 or 3, comprising: obtaining a fourth measurement parameter value from a fourth sub-target; determining a first signal based on a difference and/or comparison between the fourth measurement parameter value and the first measurement parameter value, the first signal being normalized by a difference and/or comparison between the fourth measurement parameter value and the third measurement parameter value; determining a second signal based on a difference and/or comparison between the fourth measurement parameter value and the second measurement parameter value, the second signal being normalized by a difference and/or comparison between the fourth measurement parameter value and the third measurement parameter value; and determining the focus parameter based on the first signal and the second signal. 5. The method of clause 4, wherein the determining step further comprises determining a dose parameter based on the first signal and the second signal. 6. The method of clause 5, wherein the determining step comprises determining the dose parameter and focus parameter that minimize a mismatch error between optimized signal data and monitoring signal data, wherein the optimized signal data comprises optimized first signal data and optimized second signal data that change at least the focus parameter, and the monitoring signal data comprises the first signal and the second signal. 7. The method of clause 6, wherein the optimized signal data comprises optimized first signal data and optimized second signal data that change the focus parameter and the dose parameter. 8. The method of any one of clauses 4 to 7, wherein the first sub-goal, the second sub-goal, the third sub-goal, and the fourth sub-goal are all contained within a single target and can be measured in a single acquisition. 9. The method of any one of clauses 4 to 8, comprising an initial target selection step of selecting each of the first sub-goal, the second sub-goal, the third sub-goal, and the fourth sub-goal from a library of candidate sub-goals. 10. The method of clause 9, wherein the target selection step comprises: obtaining a library of candidate sub-targets; obtaining measurement parameter data, the measurement parameter data comprising respective measurement parameter values for each candidate sub-target in the library within a focus range of interest and a dose range of interest; fitting the measurement parameter values in focus and dose for each candidate sub-target based on a plurality of fitting coefficients; determining a plurality of difference coefficients for different pairs of the candidate sub-targets based on the plurality of fitting coefficients; and The plurality of difference coefficients are evaluated relative to one or more criteria to determine one or more feasible goals, each feasible goal comprising a feasible combination of four candidate subgoals to be used as the first subgoal, the second subgoal, the third subgoal, and the fourth subgoal. 11. The method of clause 10, wherein each difference coefficient is determined based on a difference between a fitting coefficient associated with a first candidate subgoal in each pair of candidate subgoals and a fitting coefficient associated with a second candidate subgoal in each pair of candidate subgoals. 12. The method of clause 10 or 11, wherein the fitting step comprises a two-dimensional, second-order fitting such that the plurality of fitting coefficients comprise a zero-order focusing term coefficient, a first-order focusing term coefficient, and a second-order focusing term coefficient. 13. The method of clause 12, wherein: the zero-order focusing term coefficients include a constant coefficient and a dose coefficient, the first-order focusing term coefficients include a focus coefficient and a focus-dose product coefficient; and the second-order focusing term coefficients include a focus square coefficient and a focus square-dose product coefficient. 14. The method of clause 12 or 13, wherein the difference coefficients include a first difference coefficient determined based on the zero-order focusing term coefficients, a second difference coefficient determined based on the first-order focusing term coefficients, and a third difference coefficient determined based on the second-order focusing term coefficients. 15. The method of claim 14, wherein: the first difference coefficient is determined based on a difference between the zero-order focusing term coefficients of a first candidate sub-target in each pair of candidate sub-targets and the zero-order focusing term coefficients of a second candidate sub-target in each pair of candidate sub-targets; the second difference coefficient is determined based on a difference between the first-order focusing term coefficients of a first candidate sub-target in each pair of candidate sub-targets and the first-order focusing term coefficients of a second candidate sub-target in each pair of candidate sub-targets; the first difference coefficient is determined based on a difference between the second-order focusing term coefficients of a first candidate sub-target in each pair of candidate sub-targets and the second-order focusing term coefficients of a second candidate sub-target in each pair of candidate sub-targets; and wherein each dose-related focusing term coefficient is multiplied by a nominal dose value. 16. The method of clause 14 or 15, wherein the evaluating step comprises determining a feasible combination of the first sub-goal, the second sub-goal, and the third sub-goal by evaluating, for different combinations of the first and second pairs of the candidate sub-goals, whether a difference between the respective second difference coefficients for the first pair of the candidate sub-goals and the second pair of the candidate sub-goals is not less than a second difference coefficient threshold value. 17. The method of clause 14, 15, or 16, wherein the evaluating step comprises determining, for different combinations of the first and second pairs of the candidate sub-goals, whether a difference between the respective first difference coefficients for the first pair of the candidate sub-goals and the second pair of the candidate sub-goals is not greater than a first difference coefficient threshold value. 18. The method of clause 16 or 17, wherein the evaluating step comprises determining a feasible combination of the first sub-goal and the fourth sub-goal by evaluating whether the first variance coefficient for each pair of the candidate sub-goals is not less than a normalized first variance coefficient threshold value. 19. The method of clause 16, 17, or 18, wherein the evaluating step comprises determining a feasible combination of the first sub-goal and the fourth sub-goal by: evaluating whether the second variance coefficient for each pair of the candidate sub-goals is not greater than a normalized second variance coefficient threshold value; and evaluating whether the third variance coefficient for each pair of the candidate sub-goals is not greater than a normalized third variance coefficient threshold value. 20. The method of clause 18 or 19, comprising combining the determined feasible combination of the first sub-goal and the fourth sub-goal with the determined feasible combination of the first sub-goal, the second sub-goal, and the third sub-goal to determine the one or more feasible goals. 21. The method of any one of clauses 10 to 20, comprising selecting at least one of the one or more feasible goals. 22. The method of clause 21, wherein the selecting step comprises: determining a focus acquisition setting error and a focus sensitivity for each of the one or more feasible goals; and selecting a goal from the one or more feasible goals in which the focus sensitivity is not less than a focus sensitivity threshold and/or a focus acquisition setting error is not less than a focus acquisition setting error threshold. 23. The method of clause 22, wherein the selecting step further comprises: determining a dose-acquisition setting error and a dose sensitivity for each of the one or more feasible targets; and selecting a target from the one or more feasible targets for which the dose sensitivity is not less than a dose sensitivity threshold and/or a dose-acquisition setting error is not less than a dose-acquisition setting error threshold. 24. The method of any preceding clause, wherein the first sub-target or a combination of the first sub-target and the second sub-target comprises a focus sensitivity greater than a focus sensitivity threshold. 25. The method of any preceding clause, wherein at least the third sub-target comprises a focus sensitivity less than a focus sensitivity threshold. 26. The method of any preceding clause, wherein each of the sub-objectives is substantially symmetric. 27. The method of any preceding clause, wherein the first measurement parameter value describes a focusing-dependent asymmetry of the first sub-objective. 28. The method of any preceding clause, wherein each of the measurement parameter values comprises a strength metric. 29. The method of clause 28, wherein the strength metric comprises an average of respective strengths of complementary diffraction orders from each of the sub-objectives. 30. The method of any preceding clause, comprising measuring a target to obtain each of the measurement parameter values. 31. The method of any preceding clause, wherein the third sub-objective comprises a dedicated normalization sub-objective such that measurement data from the normalization sub-objective is used for the sole purpose of normalization. 32. A method for designing a focus target, comprising: obtaining a library of candidate sub-targets; obtaining measurement parameter data, the measurement parameter data comprising respective measurement parameter values for each candidate sub-target in the library within a focus range of interest and a dose range of interest; fitting the measurement parameter values in focus and dose for each candidate sub-target based on a plurality of fitting coefficients; determining a plurality of difference coefficients for different pairs of the candidate sub-targets based on the plurality of fitting coefficients; and evaluating the plurality of difference coefficients relative to one or more criteria to determine one or more feasible targets, each feasible target comprising a feasible combination of candidate sub-targets. 33. The method of clause 32, wherein each difference coefficient is determined based on a difference between a fitting coefficient associated with a first candidate sub-target in each pair of candidate sub-targets and a fitting coefficient associated with a second candidate sub-target in each pair of candidate sub-targets. 34. The method of clause 32 or 33, wherein the fitting step comprises a two-dimensional, second-order fitting, such that the plurality of fitting coefficients comprises a zero-order focusing term coefficient, a first-order focusing term coefficient, and a second-order focusing term coefficient. 35. The method of clause 34, wherein: the zero-order focusing term coefficients include a constant coefficient and a dose coefficient, the first-order focusing term coefficients include a focus coefficient and a focus-dose product coefficient; and the second-order focusing term coefficients include a focus square coefficient and a focus square-dose product coefficient. 36. The method of clause 34 or 35, wherein the difference coefficients include a first difference coefficient determined based on the zero-order focusing term coefficients, a second difference coefficient determined based on the first-order focusing term coefficients, and a third difference coefficient determined based on the second-order focusing term coefficients. 37. The method of claim 36, wherein: the first difference coefficient is determined based on a difference between the zero-order focusing term coefficients of a first candidate sub-target in each pair of candidate sub-targets and the zero-order focusing term coefficients of a second candidate sub-target in each pair of candidate sub-targets; the second difference coefficient is determined based on a difference between the first-order focusing term coefficients of a first candidate sub-target in each pair of candidate sub-targets and the first-order focusing term coefficients of a second candidate sub-target in each pair of candidate sub-targets; the first difference coefficient is determined based on a difference between the second-order focusing term coefficients of a first candidate sub-target in each pair of candidate sub-targets and the second-order focusing term coefficients of a second candidate sub-target in each pair of candidate sub-targets; and wherein each dose-related focusing term coefficient is multiplied by a nominal dose value. 38. The method of any one of clauses 32 to 37, wherein each feasible goal comprises a feasible combination of a first sub-goal, a second sub-goal, a third sub-goal, and a fourth sub-goal. 39. The method of clause 38, wherein the evaluating step comprises determining the feasible combination of the first sub-goal, the second sub-goal, and the third sub-goal by evaluating, for different combinations of the first and second pairs of the candidate sub-goals, whether a difference between the second difference coefficients for the first pair of the candidate sub-goals and the second pair of the candidate sub-goals is not less than a second difference coefficient threshold value. 40. The method of clause 38 or 39, wherein the evaluating step comprises determining feasible combinations of the first sub-goal, the second sub-goal, and the third sub-goal for different combinations of the first and second pairs of the candidate sub-goals by evaluating whether a difference between the first difference coefficients for a first pair of the candidate sub-goals and a second pair of the candidate sub-goals is no greater than a first difference coefficient threshold value. 41. The method of clause 39 or 40, wherein the evaluating step comprises determining feasible combinations of the first sub-goal and the fourth sub-goal by evaluating whether the first difference coefficient for each pair of the candidate sub-goals is no less than a normalized first difference coefficient threshold value. 42. The method of clause 39, 40, or 41, wherein the evaluating step comprises determining a feasible combination of the first sub-goal and the fourth sub-goal by: evaluating whether the second variance coefficient for each pair of the candidate sub-goals is no greater than a normalized second variance coefficient threshold; and evaluating whether the third variance coefficient for each pair of the candidate sub-goals is no greater than a normalized third variance coefficient threshold. 43. The method of clause 41 or 42, comprising combining the determined feasible combination of the first sub-goal and the fourth sub-goal with the determined feasible combination of the first sub-goal, the second sub-goal, and the third sub-goal to determine the one or more feasible goals. 44. The method of any of clauses 32 to 43, comprising selecting at least one of the one or more feasible goals. 45. The method of clause 44, wherein the selecting step comprises: determining a focus acquisition setting error and a focus sensitivity for each of the one or more feasible targets; and selecting a target from the one or more feasible targets in which the focus sensitivity is not less than a focus sensitivity threshold and/or a focus acquisition setting error is not less than a focus acquisition setting error threshold. 46. The method of clause 45, wherein the selecting step further comprises: determining a dose acquisition setup error and a dose sensitivity for each of the one or more feasible targets; and selecting a target from the one or more feasible targets for which the dose sensitivity is not less than a dose sensitivity threshold and/or a dose acquisition setup error is not less than a dose acquisition setup error threshold. 47. The method of any of clauses 32 to 46, wherein each of the candidate targets is substantially symmetric. 48. The method of any of clauses 32 to 47, comprising: exposing a focus target on a substrate according to at least one of the feasible targets. 49. The method of clause 48, comprising measuring the focus target to determine a focus setting for exposing the focus target. 50. A reticle comprising focus target features configured to pattern a radiation beam, the focus target features corresponding to the at least one of the possible targets as determined by the method of any of clauses 32 to 47. 51. A substrate comprising at least one focus target, the at least one focus target corresponding to the at least one of the possible targets as determined by the method of any of clauses 32 to 47. 52. A computer program comprising program instructions operable to perform the method of any of clauses 1 to 47 when executed on a suitable apparatus. 53. A non-transitory computer program carrier comprising the computer program of clause 41. 54. A processing system comprising a processor and a storage device comprising the computer program of clause 41. 55. A lithography apparatus comprising the processing system of clause 54. 56. A non-transitory computer program product comprising machine-readable instructions which, when executed by a computer system, are configured to cause the computer system to at least cause the method of any one of clauses 1 to 47 to be performed. 57. A metrology apparatus operable to perform the method of any one of clauses 1 to 47. 58. A metrology target for determining a parameter of a lithography process, the target comprising at least a first sub-target, a second sub-target, a third sub-target, and a fourth sub-target, wherein each of the sub-targets has a design different from that of the other targets, wherein the sub-targets include two substantially sensitive sub-targets for the parameter, and two substantially insensitive sub-targets for the parameter, and wherein the parameter of the lithography process is determined based on metrology parameter data of the first sub-target, the second sub-target, the third sub-target, and the fourth sub-target. 59. A method for designing a goal for measuring a first parameter, wherein the goal includes multiple sub-goals, the method comprising: obtaining a library of candidate sub-goals; obtaining measurement parameter data, the measurement parameter data including respective measurement parameter values for each candidate sub-goal in the library of candidate sub-goals within a range of interest of the first parameter and a range of interest of a second parameter; wherein the measurement parameter data includes a matrix that varies as a function of the first parameter and the second parameter; and determining one or more feasible goals, each feasible goal including a feasible combination of candidate sub-goals, such that the matrix has a single global minimum or maximum value within the range of interest of the first parameter and the range of interest of the second parameter. 60. The method of clause 59, wherein the target is used to measure the first parameter and the second parameter. 62. The method of clause 61, wherein the first parameter is determined based on a difference and/or comparison between the first and second measurement parameter values. 63. The method of clause 61 or 62, wherein the target further comprises a third sub-target and the measurement parameter data, the measurement parameter data comprising a third measurement parameter value associated with at least the third sub-target. 64. The method of clause 63, wherein the target further comprises a fourth sub-target and the measurement parameter data comprising a fourth measurement parameter value associated with at least the fourth sub-target. 65. The method of clause 64, wherein the sub-targets include two sub-targets that are substantially sensitive to the first parameter and two sub-targets that are substantially insensitive to the first parameter. 66. The method of any of clauses 59 to 65, wherein the first parameter is a parameter of a lithography process. 67. The method of clause 66, wherein the first parameter is a focus parameter. 68. The method of clause 66 or 67, wherein the method further comprises using a computational lithography module capable of simulating the lithography process of transferring a patterned device pattern onto a resist layer on a substrate, and a computational metrology module capable of simulating metrology operations on the patterned device pattern, wherein the patterned device pattern is used to pattern the target. 69. The method of any of clauses 59 to 68, wherein the second parameter is a dose parameter. 70. The method of any of clauses 59 to 69, wherein each of the sub-goals is substantially symmetric. 71. The method of any of clauses 59 to 70, wherein each of the sub-goals has a different design than the other goals. 72. The method of any of clauses 59 to 71, wherein all of the sub-goals can be measured in a single acquisition. 73. The method of any of clauses 59 to 72, wherein each of the measured parameter values comprises an intensity metric. 74. The method of clause 73, wherein the intensity metric comprises an average of the respective intensities of the complementary diffraction orders from each of the sub-goals. 74. A method for inferring a value of a first parameter and a value of a second parameter of a lithography process, comprising: determining a first metric and a second metric based on metrology data associated with a target formed on a substrate using the lithography process, each of the first metric and the second metric depending on both a first process parameter and a second process parameter, the first metric having a different dependency on the first process parameter than the second process parameter, and the second metric having a different dependency on the second process parameter than the first process parameter; and inferring the value of the first parameter and the value of the second parameter based on the first metric and the second metric, wherein the target comprises only symmetric features. 75. The method of clause 74, wherein the first parameter is a focus parameter. 76. The method of clause 74 or 75, wherein the second parameter is a dose parameter. 77. The method of any of clauses 74 to 76, wherein the target comprises a first sub-target and a second sub-target. 78. The method of clause 77, wherein the first sub-target and the second sub-target have different designs. 79. The method of clause 78, wherein the first sub-target and the second sub-target have different respective best focus values. 80. The method of any of clauses 74 to 79, wherein the first metric and/or the second metric is an intensity metric asymmetry between the first sub-target and the second sub-target. 81. The method of any of clauses 74 to 80, wherein each of the metrics comprises a sum of intensities from each of two complementary diffraction orders of each respective sub-target. 82. The method of any of clauses 74 to 81, wherein the lithography process comprises using extreme ultraviolet radiation for exposure. 83. A non-transitory computer program product comprising machine-readable instructions which, when executed by a computer system, are configured to cause the computer system to at least cause the method of any one of clauses 59 to 82 to be performed.

Although specific reference may be made herein to the use of lithography equipment in IC manufacturing, it should be understood that the lithography equipment described herein may have other applications. Possible other applications include the fabrication of integrated optical systems, guide and detection patterns for magnetic resonance memory, flat panel displays, liquid crystal displays (LCDs), thin film magnetic heads, and the like.

Although specific reference may be made herein to embodiments of the present invention in the context of lithography equipment, embodiments of the present invention may be used in other equipment. Embodiments of the present invention may form part of reticle inspection equipment, metrology equipment, or any equipment that measures or processes objects such as wafers (or other substrates) or reticles (or other patterned devices). Such equipment may generally be referred to as lithography tools. Such lithography tools may utilize vacuum conditions or ambient (non-vacuum) conditions.

Where the context permits, embodiments of the present invention may be implemented in hardware, firmware, software, or any combination thereof. Embodiments of the present invention may also be implemented as instructions stored on a machine-readable medium that can be read and executed by one or more processors. A machine-readable medium may include any mechanism for storing or transmitting information in a form readable by a machine (e.g., a computing device). For example, a machine-readable medium may include read-only memory (ROM); random access memory (RAM); magnetic storage media; optical storage media; flash memory devices; electrical, optical, acoustic, or other forms of propagated signals (e.g., carrier waves, infrared signals, digital signals, etc.); and others. Furthermore, firmware, software, routines, and instructions may be described herein as performing certain actions. However, it should be understood that such descriptions are for convenience only and that such actions are in fact caused by a computing device, processor, controller, or other device executing the firmware, software, routines, instructions, etc., and in doing so, may cause actuators or other devices to interact with the physical world.

Although specific embodiments of the present invention have been described above, it will be appreciated that the present invention may be practiced in other ways than those described. The foregoing description is intended to be illustrative rather than restrictive. Thus, it will be apparent to those skilled in the art that modifications may be made to the present invention as described without departing from the scope of the claims set forth below.

1: Laser system 2: Laser beam 3: Fuel emitter 4: Plasma formation region 5: Collector 6: Intermediate focus 7: Tin plasma 8: Opening 9: Enclosure 10: Faceted field mirror device 11: Faceted pupil mirror device 13: Mirror 14: Mirror 700: Candidate sub-target library 705: Step 710: Step 715: Step 720: Step 725: Step 800: Step 805: Step 810: Step 815: Step 820: Step 825: Step A: Fourth sub-target B: EUV radiation beam/first sub-target B': Patterned EUV radiation beam BK: Bake plate C: Second sub-target CD _p : Key dimensions of similar products CH: Cooling plate CL: Computer system D: Third sub-target DE: Developer df: Optimal focus offset DS: Dose sensitivity FS: Focus sensitivity IL: Illumination system I/O1: Input/output port I/O2: Input/output port k : Second candidate sub-target LA: Lithography equipment LACU: Lithography control unit LB: Loading area LC: Lithography unit m : First candidate sub-target MA: Patterning device MT: Support structure PEB: Post-exposure bake step _Pop : Optical pitch _Pp : Segment spacing PS: Projection system RO: Substrate handler or robot SC: Spin coater SC1: First scale SC2: Second scale SC3: Third scale SCS: Supervisory control system SO: Radiation source T1: First sub-target T2: Second sub-target T3: Third sub-target T4: Normalization sub-target TCU: Coating and development system control unit W: Substrate WT: Substrate stage

Embodiments of the present invention will now be described by way of example only with reference to the accompanying schematic drawings, in which: - FIG. 1 depicts a lithography system comprising a lithography apparatus and a radiation source; - FIG. 2 depicts a schematic overview of a lithography unit; - FIG. 3 depicts a schematic representation of overall lithography illustrating the cooperation between three key technologies for optimizing semiconductor manufacturing; - FIG. 4(a) and FIG. 4(b) each depict a proposed target design comprising at least one specified normalized sub-target according to a first embodiment; - FIG. 5 depicts a proposed target design comprising at least one specified normalized sub-target according to a second embodiment; - FIG. 6(a), FIG. 6(b) and FIG. 6(c) each depict possible candidate sub-target variations that can be used in a target design method according to one embodiment; FIG7 is a flowchart illustrating a target design method according to one embodiment; and - FIG8 is a flowchart illustrating the final step of the target design method of FIG7 according to one embodiment.

T1: First sub-goal

T2: Second sub-goal

T3: The third sub-goal

T4: Regularization sub-goal

Claims (15)

A metrology method comprises: obtaining a first measurement parameter value associated with at least a first sub-target; obtaining a third measurement parameter value associated with at least a third sub-target; and determining at least one focus parameter based on at least the first measurement parameter value, wherein the determination is normalized using at least the third measurement parameter value, wherein the at least third sub-target includes a normalized sub-target, wherein the normalized sub-target includes substantially no focus sensitivity or includes minimal focus sensitivity. The method of claim 1, further comprising obtaining a second measurement parameter value associated with at least one second sub-target; and determining at least one focus parameter based on the first measurement parameter value and the second measurement parameter value. The method of claim 2, wherein the focusing parameter is determined based on a difference and/or comparison between the first measurement parameter value and the second measurement parameter value. The method of claim 2 or 3, comprising: obtaining a fourth measurement parameter value from a fourth sub-target; determining a first signal based on a difference and/or comparison between the fourth measurement parameter value and the first measurement parameter value, the first signal being normalized by a difference and/or comparison between the fourth measurement parameter value and the third measurement parameter value; determining a second signal based on a difference and/or comparison between the fourth measurement parameter value and the second measurement parameter value, the second signal being normalized by a difference and/or comparison between the fourth measurement parameter value and the third measurement parameter value; and determining the focus parameter based on the first signal and the second signal. The method of claim 4, wherein the determining step comprises further determining a dose parameter based on the first signal and the second signal. The method of claim 4, wherein the first sub-goal, the second sub-goal, the third sub-goal, and the fourth sub-goal are all contained in a single goal and can be measured in a single acquisition. The method of claim 4, comprising an initial target selection step of selecting each of the first sub-target, the second sub-target, the third sub-target, and the fourth sub-target from a candidate sub-target library. The method of claim 2 or 3, wherein the first sub-target or a combination of the first sub-target and the second sub-target comprises a focus sensitivity greater than a focus sensitivity threshold. The method of any of claims 1 to 3, wherein at least the third sub-target comprises a focus sensitivity below a focus sensitivity threshold. The method of any one of claims 1 to 3, wherein each of the sub-goals is substantially symmetric. The method of any of claims 1 to 3, wherein the first measurement parameter value describes a focus-dependent asymmetry of the first sub-objective. The method of any of claims 1 to 3, wherein the third sub-goal comprises a dedicated normalization sub-goal such that measurement data from the normalization sub-goal is used for the sole purpose of normalization. A non-transitory computer program product comprising machine-readable instructions which, when executed by a computer system, are configured to cause the computer system to at least perform the method of any one of claims 1 to 12. A metrology apparatus operable to perform the method of any one of claims 1 to 12. A metrology target for determining a parameter of a lithography process, the target comprising at least a first sub-target, a second sub-target, a third sub-target, and a fourth sub-target, wherein each of the sub-targets has a design different from that of the other targets, wherein the sub-targets include two sub-targets that are substantially sensitive to the parameter and two sub-targets that are substantially insensitive to the parameter, and wherein the parameter of the lithography process is determined based on metrology parameter data of the first sub-target, the second sub-target, the third sub-target, and the fourth sub-target.